This invention is directed to a voice processing system for amplitude modulated radio telephones and in particular to a system having a compressor attached to the transmitter and an expander attached to the receiver, the compressor and the expander being synchronized by a digital signal transmitted along with the voice.
HF radio telephones are used for communication beyond the line-of-sight. Many ships and aircraft are dependent upon HF radio telephones for vital communications, however voice traffic on HF radio telephones is often hampered by high ambient voice and by fading. In addition, HF radio telephones are deployed in remote sparsely populated regions in which satellite and troposcatter systems cannot be economically supported.
The principle of this invention is related to a system called "Lincompex" described in the publication by Carter et al "Lincompex": A system for Improving High Frequency Radio Circuits -- British Communications Electronics -- August 1965, Vol. 12, No. 8, Pages 494-497. Lincompex was developed by the British Post Office to improve the performance of radio telephones when fading and noise is encountered. Although Lincompex has proven to be successful in long range international radio telephone circuits, radio telephone users with limited means have not been able to take advantage of it for the following reasons: The Lincompex was developed over a decade ago and is expensive compared to a standard commercial radio telephone. In addition, the Lincompex requires a frequency source with stability which is significantly better than those normally supplied to inexpensive radio telephones. The capacity of the channel needed to carry Lincompex synchronization information is large since the gain can be anywhere within a 60 db range and gain change can occur at any time. When this channel fails in a noisy environment, synchronization is lost and the Lincompex system becomes inoperative.
When speech is being transmitted the peak to average power of a standard amplitude modulated SSB radio telephone is about 14.5 db. This figure is obtained when saturation of the power stage is allowed about 1% of the time. This above number implies that the maximum power capability of the transmitter is rarely fully utilized when voice is being transmitted. When noise is encountered, the loud syllables of the voice can be understood but the soft syllables are masked, degrading the intelligibility of transmitted voice.